The present invention relates to a detector for the gap of the spark plug in use for automobile internal-combustion engine, and more particularly to an apparatus of the type which can measure the spark gap by electrical and electronical means without any need of removing the spark plug from the engine.
It is well known that the spark plug provides sparks to ignite and burn the air-fuel mixture compressed in the cylinder of the engine. In order to ensure the sparking action of the spark plug or obtain the sparks at a proper timing, it is necessary to previously adjust the spark gap of the spark plug to a desired spacing.
It is also known that if the spark gap is improperly provided, that is to say, in too narrow or wide a spacing, it may lead to failure to fire in the cylinder or improper spark timing. In order to avoid such inconveniences, therefore, it has been the practice that the spacing of the gap is very often measured and adjusted as necessary during the assembly line or periodical inspection and maintenance of the engine.
There is a conventional method of measuring the spark gap of the spark plug in which the plug is usually removed from the engine so that the gap may be directly measured by means of a thickness or clearance gauge. This may provide a high-precision measuring means, but is particularly disadvantageous since it always necessitates removal of the spark plug from the engine for inspection, thus delaying the assembly work of the engine and adversely affecting the periodical inspection of the vehicle.
There is known another conventional and improved method in which a high-potential d.c. voltage or pulse voltage is supplied across the spark plug, and its spark firing voltage is measured so as to know the spark gap which is usually obtained from the Paschen's law. This allows the spark gap to be measured with the spark plug installed in the engine, thus facilitating the measuring work for that matter. It is nevertheless disadvantageous since the spark firing voltages may have significant deviations due to the possibly contaminated or oxidized electrodes of the spark plug, which may also adversely affect the high-precision measurement of the gap.
The present invention has solved these disadvantages by providing a means of measuring the spark gap with a higher precision without any need of removing the spark plug from the engine. The invention is essentially based on the observation that the portion of induced discharge voltage in the secondary voltage of the ignition system has a duration of time corresponding to the length of the spark gap.